1. Field of the Invention
The present invention relates generally to a plant for remediating contaminated soil and, more particularly, to a soil remediation plant wherein contaminated particulate matter may become entrained in and exhausted with a hot gas stream generated by the plant.
2. Description of the Related Art
Remediation of contaminated soil is, and will continue to be for the foreseeable future, a large and growing industry. For example, soil containing hydrocarbons due to service station ground leakage must be cleaned or removed before ownership of the property can be transferred. The generally applied method of remediating contaminated soil is to heat the soil with a large combustion heater in an inclined rotating drum that causes the contaminated soil, after being introduced into an input end of the drum, to be gravitationally urged toward and discharged from a discharge end of the drum. As the soil is processed through the drum, the soil is heated to a sufficiently elevated temperature by a flame and hot gas stream from the combustion heater whereby volatile contaminants in the soil--typically short-chain hydrocarbons--are released or vaporized from the contaminated soil such that the soil is remediated. The remediated soil is then removed from the rotating drum and cooled for reuse.
The hot gas stream from the combustion burner, including combustion products from the flame of the burner and the vaporized contaminants, are exhausted from the drum. Those exhausted gases and vapors are usually directed, after filtering, into an afterburner. The afterburner generally comprises a second large burner that, in conjunction with the combustion properties of the contaminates, further elevate the temperature of the vapors and gases in order to break down and oxidize the contaminants remaining therein. Short-chain or light hydrocarbons, such as those arising from service station leakage, vaporize at approximately 500.degree. F. and require afterburning at approximately 1600.degree. F.
As the hot gas stream passes through the drum and interacts with the contaminated soil being processed therethrough, airborne-prone particulate matter from the contaminated soil becomes entrained in and swept along by the hot gas stream. As a result, entrained particulate matter is exhausted from the drum along with the hot gases. During the filtering process, either in a cyclone separator, baghouse filter or a combination of the two, the particulate matter is removed from the exhausted hot gases. Although the afterburner is designed to oxidize and render harmless the vaporized contaminants, the particulate matter is not so treated and continues to be a source of undesirable contamination. Therefore, the particulate matter, sometimes referred to as dust or "fines", is generally added back to the contaminated soil being processed in the drum to undergo further oxidation.
Unfortunately, by the time the fines have been filtered from the hot gas stream and returned to the drum, the temperature of those fines has generally cooled to approximately 250.degree.-300.degree. F. The temperature of those cooler fines being added back into the drum needs to be elevated to the temperature of the soil being remediated. As the fines can constitute a substantial portion of the total soil/material load being processed in the drum, 20-30% for example, heating of those fines represents a major loss of efficiency in terms of reduced output in tons per hour of the plant. In fact, the greater the percentage of fines, the greater the loss in efficiency.
Due to the extra thermal energy required to decontaminate the cooled fines added back to the drum, the combustion burner must be operated at a higher thermal output. Operating the combustion burner at a higher thermal output, in turn, generally creates a more vigorous hot gas stream which increases entrainment and further compounds the fines problem.
Further, the drum is necessarily hot in order to provide the relatively high temperatures needed to remediate the contaminated soil. As a result, substantial thermal energy is wasted due to heat loss through the cylindrical wall of the drum to the surrounding ambient atmosphere.
What is needed is a soil remediation system and apparatus for a soil remediation plant wherein particulate matter entrained in and exhausted with a hot gas stream from a rotary drum can be decontaminated without substantially reducing thermal efficiency of the soil remediation plant and, preferably, wherein the decontamination of the particulate matter can be realized by utilizing some of the thermal energy otherwise lost through the drum wall to the ambient atmosphere, thereby substantially reducing or entirely eliminating the substantial additional thermal load placed on prior art soil remediation plants by adding back the contaminated fines.